It is well known to provide a liquid crystal display (LCD) device with a CCFL backlight. In order to provide a sufficient and uniform brightness over a large display area, such as may be necessary for example for a television display, the backlight typically comprises a plurality of CCFLs. For example, an LCD display device may comprise a plurality of CCFLs arranged horizontally parallel to and one above another behind an LCD so as to extend over substantially all of the area of the LCD, with reflectors behind the CCFLs, and a diffusion screen, polarizing filter, and colour filters between the CCFLs and the LCD. In such an arrangement there may be a substantial number of CCFLs, for example from 16 to 36 for a large display area.
A CCFL, like other discharge tubes, has a breakover or ignition voltage which must be exceeded in order to establish a discharge. A voltage-current characteristic of the CCFL is such that, after establishment of a discharge, voltage falls to a valley voltage with increasing current, and then increases with increasing current in a normal operating region in which the voltage may be appreciably less than the breakover voltage. For example, the breakover voltage of a CCFL supplied with a substantially sinusoidal AC voltage may be of the order of 1500 volts, and the operating voltage may be of the order of 700 to 800 volts, with an operating current, at this operating voltage, of the order of 5 to 10 mA. A slope of the voltage-current characteristic in the operating region is referred to as the impedance of the plasma or of the CCFL.
A result of this characteristic is that a plurality of CCFLs can not simply be connected together directly in parallel to a single power supply, because initiation of the discharge in one of the CCFLs would prevent a discharge being established in other CCFLs. A ballast resistor can be connected in series with each of the CCFLs to facilitate such a parallel connection to a single power supply, but this would result in an unacceptable power loss in the ballast resistors. Alternatively, a ballast capacitor can be connected in series with each of the CCFLs to facilitate such a parallel connection to a single power supply.
With either ballast resistors or capacitors, inevitable differences between the characteristics of the individual CCFLs result in different backlight levels over the area of the display. Such arrangements do not provide any individual control of the CCFLs, as may also be desired for example to provide different brightnesses in different areas of the display device.
It is known to provide an individual inverter (DC to AC power converter) for supplying AC power to each CCFL of a display device, and to provide a single DC to DC converter whose output voltage is supplied to all of the inverters. For example this DC to DC converter may have an output voltage in a range from 12 to 24 volts, and may also serve to supply DC power to other parts of apparatus, for example a television receiver, including the display device. A respective current sensing resistor can be connected in series with each CCFL to provide a feedback signal to the respective inverter, thereby to control the output of the inverter so that all of the CCFLs provide substantially the same backlight level. However, this involves the disadvantage that the display device requires as many separate inverters as it has CCFLs, which can be a significant number especially for large displays.
In such a known power supply arrangement, for example the single DC to DC converter may be supplied with power from an AC supply via a PFC (power factor correction) circuit which provides a DC output voltage of the order of, for example, 400 volts. It can be appreciated that the inverters can be supplied from the output of the PFC circuit instead of from the output of a DC to DC converter, but this does not avoid the disadvantage of requiring as many inverters as the display device has CCFLs.
Consequently, there is a need for an improved power supply arrangement for CCFLs, in particular for backlights for LCD display devices.